Dark Matter

[Frenetic Pony]

So, dark matter. Another word for, "we don't know, but it looks like matter so we'll call it that until we now otherwise."

I've read papers and overviews of papers on it. I know that it originally came from the observation that a galaxies observed mass couldn't account for such structures being held together by normal general relativity equations. The conclusion was, obviously there is some extra mass we can't see (dark) that's out there, and we don't know what it is made of.

But it seems that after a lot of staring at the sky with huge telescopes that this matter, or at least missing apparent mass, doesn't seem to behave like the normal mass we are used to.

To be more specific, we knew that if there is such a thing it didn't interact much, or at all, with other matter/anything else except through gravity. Thus leading to the WIMPS hypothesis, weakly interacting massive particles. But several newer observations of galactic structures and gravitic lensing question even that. Specifically it appears the gravity from dark matter is far more evenly spread across a galaxy than such a model would indicate, and that intergalactic space, aka vacuum, aka the vast emptiness between stars is not only filled with vacuum energy but perhaps an even spread of dark matter as well.

My question is, have I missed anything in all this? What sort of consensus, if any, is there?

[Frenetic Pony]

*Random, probably stupid thought of the day you should correct me on.

String theory states that gravity, and thus gravity waves (whatever those look like) should be a lot stronger than it/they appear. And that gravity waves slip off into dimensions of the universe so tiny it's incredibly hard to notice them from empirical observation. And yet dark matter theories are based off, essentially, gravity not being a strong enough force. So... is there any possibility to that?

As I said, feel free to correct me.

[mfb]

>> But several newer observations of galactic structures and gravitic lensing question even that.
Which ones?
As Dark Matter has no strong short-ranging force, it has no good way to aggregate in smaller structures. Of course, "small" and "galaxy scale" leaves some room for discrepancies between theory and observation.

>> and that intergalactic space, aka vacuum, aka the vast emptiness between stars is not only filled with vacuum energy but perhaps an even spread of dark matter as well
Well, not as much as in galaxies, otherwise it would not be possible to measure its influence there. How do you separate a homogeneous distribution of dark matter and vacuum energy (of opposite sign)?

[Frenetic Pony]

>> But several newer observations of galactic structures and gravitic lensing question even that.
Which ones?
As Dark Matter has no strong short-ranging force, it has no good way to aggregate in smaller structures. Of course, "small" and "galaxy scale" leaves some room for discrepancies between theory and observation.

>> and that intergalactic space, aka vacuum, aka the vast emptiness between stars is not only filled with vacuum energy but perhaps an even spread of dark matter as well
Well, not as much as in galaxies, otherwise it would not be possible to measure its influence there. How do you separate a homogeneous distribution of dark matter and vacuum energy (of opposite sign)?

Here's the relevant paper, can't find the other one I was referencing. http://arxiv.org/abs/1105.3005 , summary: http://www.physorg.com/news/2012-02-dar ... space.html

[PM 2Ring]

What happens when a cloud of dark matter collapse gravitationally? Without electromagnetic interaction, I guess it's not going to heat up like normal matter, so what happens to the kinetic energy and momentum of the particles in such a cloud?

If DM can collapse gravitationally, it won't form a star, but it can act as a nucleating centre for a regular matter star, and IIRC, it's hypothesized that central galactic black holes were seeded by DM. I guess that a big clump of DM could form a BH by itself, and that an active DM BH would be dark, since a pure DM accretion disk wouldn't be able to radiate like one made of regular matter. Does that sound right?

[doogly]

Modifying gravity (by brane leaking or whatever else) will not give you the difference between center of luminous matter and center of gravity that we observe in collisions like the Bullet Cluster (and there have been more observed since that one) so some sort of matter is very much the favored proposal.

There are two important reasons for DM to be weakly interacting - - one is the obvious one, that it is not luminous. But in order to have the distribution throughout a galaxy it does, it must not have a way to shed its angular momentum. This keeps the dark matter out in the halos, because it can't fall into the center. It interacts with gravity normally, living in orbits, but those orbits don't decay.

[WarDaft]

One thing I've wondered... is there in fact reason to believe that dark matter would actually be weakly interacting? I mean, we have no practical way of detecting microhertz photons (though obviously such things would not cluster in the long term) but they would still produce a gravitational effect if you had enough of them, no?

[doogly]

What are you suggesting?
That they be photons? No, those won't cluster, as you said. They scale all wrong for this.
That they not interact at all, even weakly? But I suppose it's an option?

[WarDaft]

That they possibly be interacting only gravitationally, or interacting only at scales where we simply cannot detect it. Though the latter would be more difficult to believe, because then it would also have to be interacting only on scales so large that we cannot detect its effect even on supermassive stars. But depending on the particular effects it might have on such larger bodies, that might not be that inconceivable.

[eSOANEM]

That they possibly be interacting only gravitationally,

IIRC (from a New Scientist article a few years ago), amongst various proposed things making up dark matter (chief amongst them being the WIMPs and MaCHOs we're all familiar with), sterile neutrinos which only interacted gravitationally were proposed. No idea if anything came out of that and, as I say, the article was a while ago so I could be misremembering things or remembering an exaggerated report.

[Charlie!]

How would you make something if you can't interact with it non-gravitationally? As far as I know, making something and detecting something can only use the same set of interactions. Maybe produce it near black holes?

[starslayer]

That they possibly be interacting only gravitationally,

IIRC (from a New Scientist article a few years ago), amongst various proposed things making up dark matter (chief amongst them being the WIMPs and MaCHOs we're all familiar with), sterile neutrinos which only interacted gravitationally were proposed. No idea if anything came out of that and, as I say, the article was a while ago so I could be misremembering things or remembering an exaggerated report.

Neutrinos have been proposed, but they're still relativistic, even at 2.95 K (the temperature of the cosmic neutrino background), so they can't concentrate enough to match the observed mass distribution. They also have extremely tiny masses (like a few eV at most), so you'd need more than are probably in existence.

[eSOANEM]

Ordinary neutrinos would certainly be hard to make fit the data, but the article was suggesting that there might be a new, undiscovered, sterile neutrino which did not interact weakly (so only interacted gravitationally) and, because it has not been discovered, its mass is unknown so it could potentially be large enough for them to match the observed mass distribution.

[The Geoff]

It could, of course, be something other than dark matter. A modification to GR at large distances would do the job, I believe this is one of the aims of Modified Newtonian Dynamics (MOND)?

[doogly]

Yes, but modifying gravity would have to find some way to explain the bullet cluster (et al), where the center of luminous mass and the center of gravitational mass (as determined by lensing) do not appear to be in the same place.

[PM 2Ring]

Ordinary neutrinos would certainly be hard to make fit the data, but the article was suggesting that there might be a new, undiscovered, sterile neutrino which did not interact weakly (so only interacted gravitationally) and, because it has not been discovered, its mass is unknown so it could potentially be large enough for them to match the observed mass distribution.

I'm sceptical: why would sterile neutrino have a higher rest mass than the non-sterile varieties? A lower rest mass makes sense, though.

Neutrinos are like electrons without a charge, and IIRC the theory is that most of the rest mass of the electron is due to it's electromagnetic charge and a small amount is due to it feeling the weak force. In a sense, the electron stores electroweak energy, and that energy has mass. Alternatively, you can think of that mass coming from the cloud of virtual particles associated with the electron. Hopefully, Doogly (or someone) will correct me if I've screwed that up.

For sterile (or non-sterile) neutrinos to be suitable DM candidates you'd expect them to have velocities consistent with orbital motion, otherwise they'd tend to dissipate rather than to retain some kind of structure. But neutrinos are generally moving very close to c, and they aren't easy to slow down; sterile neutrinos would be even harder to slow down. OTOH, traveling at high speed gives them substantial relativistic mass, and I guess that could be significant gravitationally.

[yurell]

It could, of course, be something other than dark matter. A modification to GR at large distances would do the job, I believe this is one of the aims of Modified Newtonian Dynamics (MOND)?

I didn't think MOND worked relativistically, and that's why they came up with theories like TeVeS?

[SU3SU2U1]

sterile neutrinos which only interacted gravitationally were proposed.

I'm sceptical: why would sterile neutrino have a higher rest mass than the non-sterile varieties?

Generally, sterile neutrino models are motivated by a few things

1. all observed neutrinos have left-handed helicity. Why shouldn't they have a right handed partner?
2. SO(10) type grand unified theories fit the standard model nicely into a 16 dimensional representation. However, there are only 15 particles in a generation of the standard model (3 colors * 2 quarks*2 helicities+1 leptons*2 helicities+1 neutrino). Add a right handed neutrino and everything fits.
3. if you add a really majorana right handed neutrino, the mass matrix looks like this

\left(\begin{array}{cc} 0 & m_{dirac}\\m_{dirac}& m_{sterile}\end{array}\right)

If the sterile mass is very big, the eigenvalue looks like \lambda = \frac{m_{dirac}^2}{m_{sterile}}

So if the sterile neutrino takes a big mass, the neutrino eigenvalues become naturally small.

[mfb]

Here's the relevant paper, can't find the other one I was referencing. http://arxiv.org/abs/1105.3005 , summary: http://www.physorg.com/news/2012-02-dar ... space.html

Well, looking at the logarithmic y-scale and the large distances on the x-scale (Andromeda would be closer than the 1000-mark), it does not look so strange.

>> As far as I know, making something and detecting something can only use the same set of interactions.
Why? Particle accelerators produce a lot of stuff via the strong interaction, which decays via the weak interaction and the decay products are then observed via their electromagnetic interaction. Guess which interaction is missing , as gravity is just too weak. But it could produce dark matter particles. But if they don't interact weakly, we have no way to detect them as individual partices.

Neutrinos are like electrons without a charge, and IIRC the theory is that most of the rest mass of the electron is due to it's electromagnetic charge and a small amount is due to it feeling the weak force.

How does that explain the myon and tau mass?

[WarDaft]

Furthermore, it implies that the strengths of every particle's various interactions is directly proportional to its mass. Shouldn't then WIMPS have an extremely strong weak force interaction? As in about a billion times stronger than is present in a neutrino, if we assume a nutrino is a few eV and that a WIMP is on the GeV range?

[PM 2Ring]

Neutrinos are like electrons without a charge, and IIRC the theory is that most of the rest mass of the electron is due to it's electromagnetic charge and a small amount is due to it feeling the weak force.

How does that explain the myon and tau mass?

Rather poorly, I'm afraid.

Furthermore, it implies that the strengths of every particle's various interactions is directly proportional to its mass. Shouldn't then WIMPS have an extremely strong weak force interaction? As in about a billion times stronger than is present in a neutrino, if we assume a nutrino is a few eV and that a WIMP is on the GeV range?

Good point. OTOH, the weak force does have a rather short range, due to the masses of the W & Z bosons.

[WarDaft]

But we would still expect a possible detection rate a billion times the detection rate of neutrinos if neutrinos and WIMPs were equinumerous in our local area. This of course does permit WIMPs to be far less than equinumerous with nutrinos, and really suggests that we should ideally be able to detect them at rates roughly proportional to their local mass.